In seismic exploration it is known to generate seismic pulses or waves from at least one seismic source and to measure or record the produced wavefield using a plurality of seismic receivers. For marine seismic acquisition, the receivers are often arranged as streamers to be towed behind a seismic acquisition vessel. In this way, reflections, interactions or the like of the seismic pulses with earth formations may be analyzed.
In marine seismic acquisition there is typically a distance of over 100 m between the source position and the closest receiver in the seismic streamer. The gap is required because of the complex logistics of towing the sources and receivers. The source to receiver distance results in data or traces missing from the acquisition for the small offsets between source and nearest receivers in the acquired seismic data records. Currently these small and zero-offset data can only be estimated through numerical extrapolation methods.
In current acquisition methods there are source configurations known which include seismic receivers mounted on individual airguns. The purpose of these so-called near-field receivers is to acquire signals for estimating the source signature.
In U.S. Pat. No. 4,476,553 and in the European Patent EP 0066423, the entire disclosures of which are incorporated by reference herein, the use of an array of near-field hydrophones or pressure sensors arranged to measure the seismic signals generated by an array of air guns producing seismic signals in a body of water is disclosed. Using the signals obtained by the near-field hydrophones, a synthetic source signal may be derived. This derived synthetic source signal is referred to as a “notional source” and may be used to provide a way of determining the far-field signature of the array of air guns in all angular directions.
As observed by Ziolkowski et al. in: Geophysical Prospecting, 1997, 45, 611-639, and in U.S. Pat. No. 4,476,553, determination of the notional source may be complicated by sea bottom reflections and, as such, accurate marine seismic measurements using the methods described in the patents are confined to deep water seismography.
A variant of the marine seismic source described above is the TRISOR™ source used by WesternGeco Ltd. In the TRISOR™ source, a TRISOR™ marine source controller enables the air gun elements to be synchronized so as to enhance the primary pulse (peak-tuning), or the first bubble (bubble-tuning) or any other part of the composite air gun signature. TRISOR™ also allows acquisition of data from a hydrophone located near to each air gun element. Although commonly referred to as near-field hydrophones (NFH), the trace from each hydrophone is actually in the far-field of the acoustic pressure radiated from the air gun.
In U.S. Pat. No. 4,648,080 the far-field signatures of marine source arrays are determined by the downward continuation of near-field data recorded along a short signature streamer of closely spaced hydrophones, located beneath the array. A similar method is disclosed in the international published patent application WO 9410585 A1.
Other inventions, such as described in U.S. Pat. No. 5,247,486, describe methods for determining a far-field signature of a plurality of seismic source elements by measuring a near-field signature of each seismic source element and interpolating a relationship between the measured near-field signature and a measured far field signature. As disclosed, an initial near-field signature of each seismic source element and an initial far-field signature of the plurality of N seismic source elements are measured simultaneously and an operator is determined from the measurements to calculate subsequent far-field signatures. Similarly, WO-2004068170-A1 discloses a method and apparatus for directional de-signature of a seismic signal. The method includes forming a plurality of far-field signatures representative of a plurality of seismic signals having a plurality of take-off angles, associating a plurality of traces representative of a plurality of reflections of the seismic signals with the plurality of far-field signatures, and forming a plurality of de-signatured traces from the plurality of traces and the plurality of associated far-field signatures.
Further attempts to improve the source-signature estimation are described by Ed Kragh et al in First Break 18, No. 6, 260-264 (June 2000). The authors suggest to subtract an estimate of the sea-bottom reflection of the source-energy from the near-field measurements and then performing the far-field estimation using the Notional Source method in the known manner.